Cathodic protection is one of the methods used to reduce corrosion problems in metallic structures exposed to aggressive aqueous environments. It is one of the most effective techniques for corrosion control, applied in a number of industrial fields. The application thereof was first reported by Humphrey Davy in 1824, disclosing a sacrificial system for protecting copper components employed in ship hulls comprising zinc or iron plates.
On one hand, cathodic protection systems with sacrificial anodes employ metals with electronegative electrochemical potential, like zinc, aluminum, magnesium or alloys thereof to protect more noble or electropositive metals and alloys, like iron, steel, copper, titanium, etc. The potential difference between the anodic metal and the structure to be protected (i.e. cathode) provides the driving force that creates a charge flow or protection current.
A cathodic protection system with sacrificial anode comprises four main components: an anode (a metal or alloy with electronegative potential), a cathode (a structure to be protected which has a more electropositive potential than that of the anode), an electrical contact between the anode and the cathode and an electrolyte (or corrosive medium) in which the anode and the cathode are immersed.
On the other hand, impressed current cathodic protection systems employ an external source of electric power to generate a potential difference between anode and cathode that enables to provide a protection current. In this case, a metal or conductive material with high corrosion resistance, like silicon-iron alloys, graphite, MMO (Mixed Metal Oxides), and stainless steel, is used as an impressed current anode, so as to ensure proper protection system durability. FIG. 4 shows an impressed current cathodic protection system scheme.
Therefore, to provide cathodic protection to a structure it is necessary to install a predetermined anodic metal mass close to the cathode (i.e. structure) to be protected. The electrochemical potential difference between the anode and the cathode will provide a system protection current. This current will depend not only on the electric potential difference between the anode and the cathode but also on the electric/electrolytic resistance of the circuit, according to Ohm's Law.I=(Ea−Ec)/R   [1]
In turn, resistance R depends on the electric resistivity of the medium and on the geometry and proximity of the anode to the structure to be protected. The higher the value of R, the lower the current provided by the protection system. Accordingly, in order to achieve proper protection for the metallic structure, the sacrificial anodes should be located so as to obtain a protection current distribution as homogeneous as possible. In this regard, for cathodic protection of oil producing wells or water injectors/producers it is complicated to achieve a uniform current distribution along the casing length. Although for this kind of structures impressed current cathodic protection systems are usually employed, enabling to produce larges currents, the high variation of formation electric resistivity across the well often causes that the protection current cannot reach the deep casing areas exposed to corrosive formations and aquifers. FIG. 5 shows an impressed current cathodic protection installation of a hydrocarbon producing well having a heterogeneous current distribution due to the variation of formation electric resistivity.
The current distribution problems shown in FIG. 5 also occur in other type of installations like pipelines, tanks batteries and industrial facilities. Besides high initial costs, maintenance problems and vandalism, impressed current cathodic protection systems applied to hydrocarbon producing wells or water injectors/producers often create interference problems with neighboring metallic structures.
The present invention provides an impressed current anode system that provides a solution of these kinds of technical problems, as it is disclosed below.